Shoujo Kakumei Utena(Revolutionary Girl Utena)- 39 episodes, with filler.
Outlaw Star- 20 something episodes. With filler.
Short and sweet.
Long ass series, but I still enjoy them: DBZ, some of the Gundam series, etc. : priceless.
Anyway, I think it depends on what kind of story it depends on, and what they're aiming for. Some animes want to stay on the air for a while, and some are just short and had their purpose. If a series is long it doesn't mean its bad and if a series is short it doesn't mean its good.